Our invention relates generally to refractory bodies containing carbon and, more particularly, to improved refractory bodies which comprise pressed and fired mixtures of SiAlON and/or silicon nitride with elemental carbon forming the bond phase. Heretofore, the use of carbon-bonded refractories for metallurgical applications has been well-known. Such known carbon-containing refractories usually consist of a mixture of refractory grains such as aluminum oxide, zirconium oxide, clays, silicon carbide, silicon oxide, or other known refractories, and carbon from flake graphite, amorphous graphite, carbon black, coke, or like source. A carbonaceous binder derived from pitch or resin also is employed to bind together the mixture of refractory grain and carbon. It has been found that these known carbon-bonded refractories have certain advantages over conventional oxide refractories. Carbon-containing refractories are more resistant to thermal shock and it has been observed that the carbon content also prevents metal wetting and slag attack, resulting in an improved service life of the refractory bodies.
Such carbon-containing refractory bodies typically find use as crucibles for the melting and casting of ferrous and nonferrous metals; for slide gate plates in the flow control of molten metals from steel ladles to tundishes, and from tundishes to continuous casting molds; for submerged pouring nozzles in the casting of molten metal from ladles to tundishes and from tundishes to continuous casting molds; in furnace runners and troughs for transferring molten metals from furnaces to ladles; and for blast furnace bricks used in the reduction of iron ore to iron, to mention a few.
It is a known practice in the materials science/ceramics art to manufacture refractory bodies, such as a submerged pouring nozzle, for example, as a composite structure in order to increase the service life of the nozzle. The body of the nozzle may be of a carbon bonded alumina and graphite refractory material with an erosion resistant, intermediate section formed of carbon zirconia and graphite refractory. The intermediate section is in the region where the submerged nozzle is in contact with the slag/metal interface. The zirconia-graphite section exhibits improved slag erosion resistance compared with nozzles which are entirely of carbon-bonded alumnina-graphite refractory. Conventional carbon bonded zirconia and graphite slagline sleeves, while offering high erosion resistance, unfortunately often lose mechanical strength and fracture during long casting sequences. Such premature failure results in a shortened casting sequence which is uneconomical and is particularly burdensome since the nozzle must usually be replaced prior to its projected erosion life. It has been observed that the fractured sleeve still contains a large proportion of refractory which has not been eroded but becomes useless due to the decreased mechanical strength caused by a crystallographic change in the structure of known zirconia-graphite refractories. This crystallographic change results from a transition from a high temperature tetragonal structure to a low temperature monoclinic crystal structure, accompanied by approximately a 3.5% volume expansion which causes cracking and subsequent failure of the sleeve. Thus, in the case of the submerged pouring nozzle, it would be beneficial if the useful service life of a slagline sleeve could be increased by avoiding the phase transformation while, at the same time, retaining a high resistance to slag erosion during service. In other applications, such as in crucibles and slide gate plates it would be desirable to increase resistance to certain non-ferrous metals and to thermal cracking.